Coral Sand Research Articles

Experimental study on the behavior of single pile foundation under vertical cyclic load in coral sand

The bearing behaviors of piles in coral sand are critical for project safety due to the unique physical and mechanical properties of coral sand, such as high void ratio, high compressibility, high friction angle and easy to break. A series of model experiments were carried out in this study to evaluate the vertical behavior of single pile in coral sand foundation under cyclic dynamic load with the consideration of the influence of the amplitude of cyclic load, the static biased load and the number of cyclic. Load-settlement relationships, settlement-cyclic number relationships, pile tip and pile side resistances have been thoroughly investigated. Results showed that the load in coral sand foundation will transfer to the pile tip more obviously compared with the pile foundation in quartz sand. Therefore, the pile side resistance will be reduced while the corresponding pile tip resistance will be increased. Particularly, the weakening of side resistance mainly occurs in the first 500 cycles. The development of cyclic cumulative settlement is affected by the combination of the magnitude of static biased load and cyclic amplitude. The existence of static biased load is conducive to restrain the cyclic cumulative settlement when the amplitude of cyclic load is less than static load.

Laboratory Experiments and Numerical Simulation on Dynamic Response of Island Reclamation Coral Sand under Aircraft Load.

The construction of island airports on coral reefs inevitably encounters the impact load of aircraft takeoff and landing. However, previous studies have not presented a detailed description of the dynamic response of the coral sand beneath the runways of island reclamation airports under aircraft load. In the current study, the coral sand of Mischief Reef Airport in the Nansha Islands, China, was selected as the background. The pore water pressure and strain characteristics of reshaped coral sand under aircraft loads with different dynamic stress amplitudes and vibration frequencies were studied using dynamic triaxial tests. Particle discrete element software was employed to study the deformation characteristics of coral sand with different particle sizes and porosities under aircraft loads. Results show that when the dynamic stress amplitude and vibration frequency were small, the pore water pressure and strain of the coral sand samples gradually increased with the number of load cycles, and the growth rate became increasingly small. When the dynamic stress amplitude and vibration frequency were large, the axial strain of the coral sand samples increased with the vibration frequency, and the growth rate exhibited an increasing trend. The deformation of the coral sand samples increased with porosity under aircraft loading. The larger the variation range of the coral sand particle size was, the larger the coral sand deformation caused by aircraft takeoff and landing load was. These results can provide a reference for the treatment and repair of the airstrip foundation of island airports.

Effects of membrane compliance on the normal-scale triaxial cyclic liquefaction test results of saturated coral sand

Correctly evaluating the membrane penetration and compliance effects in the element liquefaction test of saturated coral sand is important for test result applications. Therefore, the penetration volume measurement and triaxial cyclic liquefaction testing of saturated coral sand and quartz sand samples with the same grading and relative density were conducted for relatively loose and dense states, and the liquefaction resistance test results before and after membrane compliance correction were analyzed. The results showed that under the same conditions, the influence of the penetration and compliance of coral sand samples was more significant than that of the quartz sand samples. The general practice of ignoring the effect of membrane compliance in normal-scale triaxial tests is inapplicable in coral sand tests and will result in relatively unsafe analysis results.

Characteristics of solid–liquid phase transformation of saturated coral sand subjected to various patterns of cyclic loading

This study conducts a series of undrained cyclic shear tests on saturated coral sand with different initial relative densities ( Dr) subjected to a 90° jump rotation of the principal stress with various initial orientations. Considering liquefiable coral sand as a fluid, an important finding is that the variation in the apparent viscosity with the number of cycles is significantly affected by Dr, the effective mean principal stress ( [Formula: see text] ), cyclic stress ratio, cyclic loading path, and cyclic loading frequency. The average flow coefficient (κ) is introduced to describe the fluidity of saturated coral sand under cyclic loading by considering the variation in the shape of the stress–strain rate curve. A positive exponential correlation is observed between κ and the excess pore water pressure (EPWP) ratio ( ru) under the given cyclic loading modes. Another significant finding is that the solid–liquid phase change time is the turning point of the apparent viscosity and the average flow coefficient gradients with ru. The inversion point’s EPWP ratio is approximately 0.9, defined as the EPWP ratio during the solid–liquid phase transformation ( ruth). The ruth value is not affected by cyclic loading mode, [Formula: see text] , and Dr.

New perspective to improve the sulfate attack resistance of mortar by coral sand

Sulfate attack is one of several chemical mechanisms of concrete deterioration. It can cause expansion, crack and mechanical property degradation of concrete due to the formation of ettringite and gypsum. A new perspective is proposed in this study to improve the sulfate attack resistance of mortar by considering the porosity characteristic of coral sand. The role of coral sand in the cement mortar when it is exposed to sulfate solution is explored comprehensively. Two kinds of sands, standard sand and coral sand, are used. The mortar sample is exposed to 5 wt % Na2SO4 solution after a hydration period of 28 days. Some supplementary cementitious materials (SCMs), such as coral sand powder, limestone powder, fly ash, are used in this study to replace cement partially. After a certain exposure time, the visual appearance, mechanical properties, length and mass change, phase composition and microstructure development of the cement mortar are investigated. The results show that the cement mortar made with coral sand has a much better resistance to sulfate attack than that made with standard sand. Although sulfate attack happens inevitably in coral mortar, the use of coral sand can largely mitigate the loss of properties of mortar, such as strength, mass and linear change, etc. In particular, after 1 year of exposure, the use of coral sand in mortar largely reduces the expansion of mortar due to sulfate attack; the linear change of coral mortar is only one-seventh in that of normal mortar. Formation of ettringite in empty spaces of coral sand and other voids explains why coral mortar has better properties than normal mortar under accelerated sulfate attack. The achievement of this study can largely improve the resistance of marine concrete to chemical attack.

Biodiversity Of Marine Yeasts Isolated From Coral Sand In Truong Sa Archipelago, Khanh Hoa Province, Vietnam

Truong Sa archipelago of Vietnam are very diverse in microorganisms, however, compared to aquatic microorganisms (sea water, sediment, etc) terrestrial microorganisms (soil, coral sand, etc) has received little attention. This study focuses on assessing the biodiversity of marine yeasts in coral sand samples collected at some islands in Truong Sa archipelago. From nine coral sand samples collected at three islands: Song Tu island (three samples), Sinh Ton island (three samples), Truong Sa island (three samples), twenty – four strains of marine yeasts were isolated. The number of marine yeast strains isolated in Truong Sa island was the highest (ten strains). Sample CS9 had the highest number of strains. These strains were grouped into eight groups based on colony and cell morphology and fourteen groups by DNA fingerprinting. The results showed that there are strains in the same group according to morphology but belong to two different groups according to fingerprinting. Otherwise, some strains have different morphology but are grouped according to fingerprinting. The fourteen yeast strains representing groups by DNA fingerprinting were closely related to fourteen different yeast species and belong to ten yeast genera (Yamadazyma, Candida, Trichosporon, Saccharomyces, Kodamaea, Rhodotorula, Rhodosporidium, Aureobasidium, Meyerozyma, Pichia). Among them, the genus Candida accounted for the highest number. This is the first study on marine yeasts in coral sand in Truong Sa archipelago, Vietnam. This study can be a premise for further studies on marine yeast in different fields such as medicine, agriculture, environment, etc.

DIVERSITAS TERIPANG (HOLOTHUROIDEA) DI ZONA INTERTIDAL PANTAI MARINA WAKATOBI

The presence of sea cucumbers does not attract much attention, but their presence greatly influences the richness of marine biota and the diversity of biota species in the waters. Live in groups or spread out and are widely found in waters with a substrate of sand, coral sand, or sand mixed with mud. The substrate for Marina beach waters is sand, sand dominated by sea grass, sand with a little Acropora type coral, and sand mixed with mud and stones. Such conditions allow the growth and development of sea cucumbers. This study aims to determine the diversity of sea cucumbers (Holothuroidea) in the intertidal zone of Marina Wakatobi beach. This type of research is an experiment with cruise methods covering an area of ​​+ 500m2 in the intertidal zone of Marina Wakatobi beach. The types of sea cucumbers found were Synapta maculata, Holothuria scabra, Holothuria atra, Actinopyga echinites, Bohadschia marmorata, Holothurian hilla. Holothuria scabra is a type of sea cucumber that has a high distribution.

Seismic dynamics of revetment breakwater built on reclaimed coral sand foundation in the SCS: insight from 1 g shaking table tests

Seismic dynamics of revetment breakwater built on reclaimed coral sand foundation in the SCS: insight from 1 g shaking table tests

Influence of spatial distribution of fine sand layers on the mechanical behavior of coral reef sand

Fine coral sand commonly distributes in coral sand artificial foundation due to the effect of gravity sorting during hydraulic process and the particle breakage of coarse particle. Considering that the significant impact of the distribution form of fine sand on the strength and deformation properties of coral sand foundation, the effect of spatial distribution of fine sand layers on the mechanical behavior of coral sand was studied. The mechanical behavior of coral sand in the consolidated undrained (CU) shear tests were simulated by discrete element models (DEM) analysis method, and the impact mechanism of fine sand interlayers on mechanical strength was analyzed. The effect of thickness and number of fine sand interlayer on the mechanical behavior and shear deformation of coral sand specimens were analyzed based on the proposed uniformity degree, a parameter that quantify the spatial distribution of fine particle interlayers. The results show that the mechanical behavior of coral sand was simulated well by DEM based on particle scanning results. Under the condition of constant fine particle content, the average coordination number of particles increased and the minimum contact force between particles decreased when fine particles were distributed in the form of interlayer. It indicates that the fine particle interlayer has a diffusion effect on stress transfer, which revealed the mechanism of the macro increasing strength of 10%–20% under the same confining pressure. A linear relationship was found between the proposed parameter uniformity and the peak deviator stress of specimens, and the impact of uniformity on the strength was particularly significant under high confining pressures.

Simulation Study of Breakage in Coral Sand Particles under Dynamic Confined Compression

This paper reports a simulation-based dynamic confined compression analysis of coral sand under an impact load using the discrete element method (DEM). The evolution of particle breakage on the mesoscale was studied by investigating force chain distributions, crack development, and breakage modes. Crushable sand particles were simulated by replacing rigid particles with flexible basic unit aggregates. The Weibull parameters were incorporated within the mesoscopic parameters of particles, and random numbers were introduced. The obtained results for particle strengths were consistent with the experimental data. The numerical results demonstrated that the force chain was significantly strengthened along the loading axis during loading and weakened during unloading. The cracks were primarily generated along the strong force chain, and particle breakage caused the strong force chain to move towards relatively more complete fragments nearby. The crack was primarily oriented along the loading axis, and its anisotropy was observed to decrease with an increase in the load. Before yielding, particle breakage in the specimen was dominated by attrition at the edges and corners. After yielding, the crack extended into the particle perpendicular to the intergranular contact surface and was deflected by the simultaneous influence of pores. The fracture and shattering of particles occurred successively, and the consequent fall-off and slip filling of the fragments promoted the deformation development.

Effects of biochar-amended alkali-activated slag on the stabilization of coral sand in coastal areas

Coral sand is widely encountered in coastal areas of tropical and subtropical regions. Compared with silica sand, it usually exhibits weaker performance from the perspective of engineering geology. To improve the geomechanical performance of coral sand and meet the requirement of foundation construction in coastal areas, a novel alkali activation-based sustainable binder was developed. The alkali-activated slag (AAS) binder material was composed of ground granulated blast-furnace slag (GGBS) and hydrated lime with the amendment of biochar, an agricultural waste-derived material. The biochar-amended AAS stabilized coral sand was subjected to a series of laboratory tests to determine its mechanical, physicochemical, and microstructural characteristics. Results show that adding a moderate amount of biochar in AAS could improve soil strength, elastic modulus, and water holding capacity by up to 20%, 70%, and 30%, respectively. Moreover, the addition of biochar in AAS had a marginal effect on the sulfate resistance of the stabilized sand, especially at high biochar content. However, the resistance of the AAS stabilized sand to wet-dry cycles slightly deteriorated with the addition of biochar. Based on these observations, a conceptual model showing biochar-AAS-sand interactions was proposed, in which biochar served as an internal curing agent, micro-reinforcer, and mechanically weak point. • AAS amended with biochar could be utilized for coral sand stabilization. • The mechanical performance of stabilized sand was improved. • High biochar content resulted in superior water holding capacity of stabilized sand. • A conceptual model illustrating biochar-AAS-sand interactions was proposed.

Mechanical behaviors of coral sand and relationship between particle breakage and plastic work

As a special geotechnical material, coral calcareous sand (CCS), also widely known as coral sand or calcareous sand, surely has similarities and differences in mechanical behaviors with terrigenous sand. As the foundation material of some important structures such as ports, aircraft runways, and marine lighthouses in the South China Sea (SCS), it is necessary and has engineering significance to study its basic mechanical properties. Taking the CCS sampled from a reclaimed land on the top of a natural coral reef in the SCS as the typical case, a series of drained and undrained triaxial shear tests were performed under conventional and high confining pressure in this study. It is found that CCS has all the characteristics that all granular materials should have. However, since the irregular particle morphology and high roughness particle surface, CCS presents a great initial shear stiffness which is significantly different from that of quartz sand. Only some CCS specimens can strictly reach the critical state, and the feature lines in the p’-q space, i.e., critical state line, and phase transformation line are not affected by the factors such as the consolidation confining pressure, initial dry density, and drainage condition. However, the critical state line in the e-lnp’ space is not unique, but dependent on the initial dry density. Based on the particle breakage measurement, a new description for the relationship between particle crushing and plastic work is proposed. It could provide an experimental basis for the development of a constitutive model considering the particle breakage for CCS.

Experimental study of interface shear characteristics between calcareous soil and steel, application to estimate axial bearing capacity of steel pile

A modified direct shear tests of interface between well-graded coral sand and coral sandy-gravel with smooth steel plate were conducted in this study under various normal stress levels. The obsevation from experimental results indicates that: (1) interface shear behavior between well-graded coral soils and steel structure follows the strain hardening rules; (2) the magnitudes of friction resistances between two materials are related to the gradation and grain size of coral soils, for coral sand limit sliding displacement is around 1mm with interface friction angle of 27.9o while for coral sandy-gravel these values are 2-3mm and 32.8o respectively; (3) interface direct shear test can be used to accurately determine friction characteristics between calcareous soil and structural material, which can effectively support analysis and design of structures built on calcareous soil.

Synergistic utilization of porous coral sand and fly ash for multifunctional engineered cementitious composites with polyethylene fibers: Intensified electromagnetic wave absorption and mechanism

The large-scale consumption of industrial solid waste for fly ash is imperative for clean material production. Among these, abundant coral sand is a potential alternative to river silica sand in construction materials. This study proposes an environmentally friendly synergistic utilization of coral sand as a natural porous aggregate that can be incorporated with fly ash and polyethylene (PE) fibers to prepare multifunctional engineered cementitious composites (ECCs) with good electromagnetic (EM) wave absorption and mechanical properties. The effects of porous coral sand, PE fibers, and fly ash on the EM parameters, hydration products, pore distribution, microstructure, and reflection loss of the ECCs were systematically investigated. The mechanical properties and environmental impact were also examined. The results showed that the synergistic integration of porous coral sand, PE fibers, and fly ash optimized the pore distribution, attenuation paths, impedance matching, multiscattering effect, nanoscale effect, and interfacial polarization of the ECCs, which were conducive to the intensification of EM wave absorption. In particular, the appropriate use of natural coral sand as porous aggregates increased the EM wave absorption, while retaining the mechanical strength and excellent tensile properties of the ECCs. Environmental impact assessment showed that the alkalis of the prepared multifunctional ECCs were solidified in the C–S–H gels, which cannot be easily ionized, thereby inhibiting environmental pollution.

Excess pore water pressure generation in saturated sandy soils subjected to various cyclic stress paths

Excess pore water pressure (EPWP) plays an essential role in determining the cyclic undrained behavior of saturated sandy soils. The influence of cyclic stress paths on the EPWP generation in saturated sandy soils has emerged as an interesting issue in recent years. By using a hollow cylinder torsional shear apparatus, this paper presents the results of a systematic experimental study on the isotropically consolidated undrained responses of saturated coral sand under the 90° jump rotation of cyclic principal stresses with various orientations. Laboratory cyclic tests showed that the patterns of EPWP generation in saturated coral sand exhibits three modes that are related to the patterns of cyclic loading and levels of cyclic stress. Moreover, the unit cyclic stress ratio (USR) defined by Chen et al. (2020) can be used as an index for distinguishing among the three modes of EPWP generation under varying cyclic loadings. Furthermore, a unique form of the relationships exists among the EPWP ratio (ru), USR, and cycle ratio (N/NL) for all cyclic loading paths considered. Following this, we propose a model for the generation of ru as a function of the USR and N/NL for application. The wide applicability of this model is validated by using independent experimental data on four siliceous sandy soils in this study and from the literature.

Experimental and Numerical Study on Dynamic Response of Underground Structure in Coral Sand Under Earthquakes

ABSTRACT The seismic performance of the liquefiable coral sand sites with underground structures is unclear. To explore the particular dynamic response of coral sand-underground structure, the shaking table tests in conjunction with numerical calculations were carried out. The soil-structure separation, frequency spectrum variation and seismic fortification of underground structures in coral sand were discussed. The results show that the soil liquefaction increases the energy dissipation of the coral sand-underground structure system. However, attention should be paid to the influence of the coral sand nonlinear development near the groundwater level on the seismic stability of the underground structure during strong earthquakes.

Experimental study on liquefaction characteristics of saturated marine coral sand based on dissipated energy evaluation

The liquefaction of saturated marine coral sand is of a great issue for the safety of ocean engineering. A series of undrained cyclic torsional shear tests on saturated coral sand are carried out to further explore the liquefaction characteristics of coral sand. Based on the energy method, the effects of relative density (D r), effective mean principal stress (), and cyclic stress ratio (CSR) on undrained responses of saturated coral sand are investigated. Two failure criteria are discussed to evaluate the dissipated energy required to cause initial liquefaction (W l). The test results show that the W l is significantly affected by D r and The generation patterns of excess pore water pressure (EPWP) and single amplitude torsional strain amplitude (γ SA) along with cumulative dissipated energy (W c) are described, and the unified generation model of both u e and γ SA along with W c are established. The liquefaction resistance curves for various D r and are strongly related to W l. Analyzing the cumulative dissipated energy required to trigger liquefaction, a modified stress level index termed CSR e is proposed to normalize the liquefaction resistance curves of saturated coral sand under different D r and The test data in literature also verify the applicability of CSR e.

Effect of using grounded coral sand on hydration and strength development of portland cement paste: Experimental and GEMS modelling investigation

The use of coral sand is mainly focusing on how to use it as aggregate in marine areas, much fewer studies have been conducted to investigate its usage as SCM (supplementary cementitious material) after being grounded into powder. In this context, the influence of CSP(coral sand powder), used as SCM, on the hydration and strength development of Portland cement paste was investigated. In order to describe the hydration process, Experiments including Isothermal calorimetry, XRD and TG analysis were used, along with the GEMS modelling system. The effect of CSP content, curing temperature and w/c were investigated in this study. The result shows that coral powder plays a nucleus effect in the process of cement hydration, which promotes the early age hydration of cement. Low coral powder substitution rate of no more than 10% can enhance the strength development of CSP concrete. However, with the increase of age and replacement rate, the coral powder shows a dilution effect which would slower the exothermic rate of the cement system. As to the hydraulic products. The results shows that the form of aluminum-containing hydration products in the coral powder-cement-based system is closely related to the content of coral powder. Apart from these, the increase of the w/c ratio is conducive to increase the hydration rate and hydration product content of coral powder-Portland cement systems. And with the increase of hydration temperature, the hydration rate of the cement system and the generation rate of hydration products are accelerated, although the main type of hydration product is not changed.

Morphology characterization and discrete element modeling of coral sand with intraparticle voids

Coral sand is the main geomaterial on tropical reefs and its constituent particles are featured with complex shapes and abundant intraparticle voids. This work presents a discrete element method (DEM)-based numerical modeling of coral sand in consideration of both the irregular shape and intraparticle voids of particles. To develop the DEM model for coral sand, the acquisition and characterization of coral sand particles are first introduced. A signed distance field-based DEM applicable to arbitrarily irregular-shaped particles is then presented, in conjunction with two versatile particle models, namely spherical harmonics (SH) and level set (LS). To generate virtual coral sand particles that conform to target morphology characteristics, the combined random field and SH approach and the remedy to elongated particles are also presented. The developed DEM model is calibrated and validated against laboratory column collapse tests, where the agreements between numerical simulations and laboratory experiments demonstrate the good validity and accuracy of DEM model for coral sand. The performance of SH and LS particle models are also compared, providing a useful reference for the selection of appropriate particle models for coral sand in practice.

A Unified Formula for Small-Strain Shear Modulus of Sandy Soils Based on Extreme Void Ratios

Small-strain shear modulus (G0) is a fundamental property required in dynamic analyses. For sandy soils, G0 may be affected strongly by particle characteristics such as uniformity coefficient (Cu), mean particle size (d50), fines content (FC), and particle shape. Based on an extensive experimental study of the mechanical behavior of coral sands, this paper proposes a new formula for predicting G0 for sandy soils with various Cu, d50, FC, and particle shapes. A notable feature of the new formula is the use of the extreme void ratios (maximum void ratio emax and minimum void ratio emin) as the indexes, which were shown to be able to account for the effects of the various factors in a simple yet collective manner. Power-law correlations were established between the minimum small-strain shear modulus G0min and emax as well as between the maximum small-strain shear modulus G0max and emin. The wide applicability of this formula was validated further using extensive data from the literature from resonant column, bender element, and torsional shear tests on siliceous, calcareous, and coral sandy soils.